Organic light-emitting diodes (OLEDs) are drawing a lot of attention as light sources, since they offer several attractive and/or unique form-factor and aesthetic advantages from a user perspective, notably direct and soft areal emission from extremely thin, and sometimes conformable, devices. Moreover, the emission color of OLEDs depends on the chemical structure of the constituent organic light-emission material, which has opened the door for a large variety of emission colors, including white, via designed chemical synthesis of the organic light-emission material.
OLEDs are currently commercially available as small-sized displays in various high-end portable applications, notably cell phones and digital cameras. There are, however, other significant and emerging applications areas, such as solid-state illumination, low-cost displays and signage, where OLEDs or alternative areal light-emission technologies could become of interest, provided that current issues with scale-up and/or cost of production can be resolved.
OLEDs are fabricated in a sandwich structure, where one, or more frequently several, organic layers are sandwiched between an electron-injecting cathode and a hole-injecting anode (VanSlyke et al. in U.S. Pat. No. 4,539,507 and Friend et al. in U.S. Pat. No. 5,247,190). When a voltage is applied between the two electrodes, electrons and holes are injected into the organic layers; these electrons and holes can subsequently recombine in an organic light-emitting layer under the generation of light. In order for the generated light to escape out of the sandwich structure, at least one of the electrodes needs to be transparent or partially transparent.
For an OLED to be efficient in transforming electric power to light emission, it is critical that the cathode/electron-injection layer exhibits a low work function (defined as the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface) (Braun et al. in U.S. Pat. No. 5,408,109). Materials with sufficiently low work function for efficient electron injection in OLEDs are unfortunately not stable under ambient oxygen and/or water, and must therefore be processed and handled under vacuum and/or inert-atmosphere conditions. Today, the cathode/electron-injection layer in OLEDs is commonly deposited by thermal evaporation under high-vacuum conditions (Reineke, S. et al. Nature, 2009, 459, 234).
For an OLED to feature a desired homogenous areal light emission, it is critical that the constituent organic layers exhibit a highly constant thickness over the entire device area. As the required thicknesses of the constituent layers in OLEDs for efficient emission are in the range of 1-100 nm, this implies that the precision in deposition of the organic layers must be very high, and that a surface roughness exceeding 10 nm rarely can be tolerated. A preferable way towards scale-up and significant lowering of production costs, which as mentioned above is anticipated to pave the way to new high-volume markets, constitutes the employment of so-called “solution-based” deposition methods; these utilize the fact that many organic compounds can be dissolved or dispersed in liquids and be processed as “inks”. It is further highly preferable if the entire device fabrication can be executed under uninterrupted ambient conditions, to avoid time-consuming and costly entries/exits into/out of, e.g., vacuum chambers. Moreover, it is also preferable if the entire fabrication of all constituent layers (electrodes and organic layer(s)) can be executed in a continuous fashion, e.g. by using a roll-to-roll procedure. Examples of scalable solution-based deposition methods that fulfil these criteria include slot-die coating, gravure printing, and flexoprinting. The commercially available OLEDs as of today are fabricated using solely expensive vacuum processing, but attempts have been made to fabricate the active layer in OLEDs from inks, using primarily inkjet-printing (J. Bharathan et al. Applied Physics Letters, 1998, 21, 2660.) and spin-coating (C. Zhang, et al. Synthetic Metals, 1994, 62, 35.), but also spray-coating (Y. Aoki, et al. Thin Solid Films, 2009, 518, 493.).
However, it is notable that it is not to be expected that the entire fabrication of an OLED can be carried out under ambient conditions using solution-based deposition methods, due to the requirement of an air-reactive cathode/electron-injection layer. Moreover, high-throughput solution-based deposition techniques, such as slot-die and spray coating, typically produce layers with significant surface roughness, thus violating the OLED requirement of extremely thin and exact layers. Finally, the existence of small “dust” particles in a typical ambient atmosphere can result in severe problems with short circuits through the ˜1-100-nm-thin films utilized in OLED devices. The light-emitting electrochemical cell (LEC) exhibits the same application advantages as the OLED, as specified in the first paragraph in this section, but features a distinctly different operational procedure due to the existence of mobile ions in the organic light-emitting layer. These mobile ions are commonly introduced into the device by blending the organic light-emitting material with an electrolyte (Pei et al. in U.S. Pat. No. 5,682,043). The mobile ions redistribute when a voltage is applied between the electrodes, and allow for the initiation of doping at the two electrode interfaces; p-type doping at the anode and n-type doping at the cathode. With time, these doped regions grow in size to make contact, so that a light-emitting p-n junction forms in the bulk of the active layer. The consequences of these in-situ doping and p-n junction formation processes are that LECs, in contrast to OLEDs, do not depend on the utilization of an air-reactive cathode/electron-injection layer and thin and exactly controlled organic layers for efficient operation. Instead, LECs can feature air-stabile and solution-processable electrodes and one thick and uneven layer as the light-emission (active) layer.
The major drawback with LECs compared to OLEDs has been a very short operational lifetime. However, recent progress in this field has resulted in that it is now possible to attain rather impressive operational lifetimes of several 1000 hours at a significant brightness of >100 cd/m2 for LEC devices (A. Asadpoordarvish, et al. Applied Physics Letters, 2012, 100, 193508.). With this in mind, it is appropriate to focus on the development of low-cost and scalable production methods of LEC devices, preferably by using solution-based deposition methods carried out under ambient conditions in a manner compatible with high-volume roll-to-roll (R2R) production.
Most current LECs are still fabricated akin to how polymer-based OLEDs are made, namely with vacuum-processed electrodes sandwiching a thin spin-coated active layer. Recent inventions have, however, demonstrated that it is possible to fabricate the active layer in planar LECs with inkjet-printing (G. Mauthner, et al. Organic Electronics, 2008, 9, 164), and the top electrode and the active layer in sandwich-cell LECs with slot-die coating (A. Sandström, et al. Nature Communications, 2012, 3, 1002), screen printing (Victor et al. in U.S. Pat. No. 7,115,216), and doctor-blade coating (Matyba et al. in Swedish patent No. SE 534,257). A drawback with spin-coating is that a large majority of the (often expensive) active-material ink is wasted during the spinning. Spin-coating and inkjet printing are in addition not easily upscalable deposition techniques. An important drawback with slot-die coating, screen printing and doctor-blade coating is that the solvent commonly exhibits a slow evaporation time, which is particularly true when thick films insensitive to damage by, e.g., ambient dust particles are to be fabricated. The slow evaporation time has the following undesired consequences: (i) It is difficult to attain multilayer structures, as the solvent in the solution-under-deposition tends to dissolve the beneath (dry) layer. This problem can be alleviated/resolved via the employment of materials with orthogonal solubility, but such a procedure puts a severe restriction on the number of available material combinations in a multilayer geometry. (ii) The light-emitting material and the electrolyte in the active layer have ample time to phase separate and/or form concentration gradients on a macroscopic scale and/or crystallize during the slow solvent evaporation, with a typical undesired manifestation being a distinctly non-homogenous light emission area. Moreover, it is very difficult to attain 2D light-emission patterns, to attain multi-colored emission, and to coat uneven surfaces using slot-die coating. Finally, all of the above introduced solution-based deposition techniques require a high solute-concentration, which further restricts the number of available materials, as many electronically active organic compounds exhibit a low or intermediate solubility, particularly in industrially preferred solvents such as water. An additional problem with a high solute-concentration solution is that it, in contrast to a low-concentration solution, is difficult to purify from impurities in the form of micron-sized “dust” particles.
Thus, it is highly desirable to establish and develop a material-conservative deposition method for LEC fabrication that can be carried out under uninterrupted ambient conditions. The method should be easily upscalable, and allow for fabrication of thick and fault-tolerant LEC-active films that can generate homogenous light emission over large surfaces when sandwiched between two similarly deposited electrode materials, at least one of which is transparent. It is further desirable if the developed deposition method can allow for deposition of low-solubility organic compounds and/or low-concentration solutions, generate 2D single- and multi-colored emission patterns, and coat complex and non-flat surfaces.
The patents and articles described in this section disclose many materials that are useful in the present invention, and their disclosures are incorporated herein by reference.